I. Field of the Invention
This invention relates generally to an electronic circuit for use in demand-type cardiac pacing apparatus, and more specifically to the design of a switching circuit used to selectively couple the pulse generator to the heart contacting electrodes during a pacing interval but disconnecting the pulse generator and connecting the R-wave sensing amplifier to the heart contacting electrodes during a sensing interval.
II. Discussion of the Prior Art
In demand-type cardiac pacers the circuitry functions to generate pulses at a desired rate for application to the heart of the patient only in the absence of naturally occurring heartbeat activity. The same electrodes which are utilized to apply the artificial stimulating pulses to the heart in the absence of naturally occurring heart activity are also used to pick up the electrical signals generated upon depolarization of the heart muscle and to apply these signals to the R-wave sensing amplifier, the output of which is used to inhibit or reset the pulse generator so that it does not produce artificial stimulating pulses when normal R-waves are being produced on a regular basis. To keep the size of the unit sufficiently small so that it may be implanted within the body of the patient, the number of battery cells employed is limited. To achieve artificial stimulating pulses of a sufficient amplitude to ensure capture, prior art pacer systems often employ a voltage doubler circuit which is disposed between the pulse generator output and the electrode lead terminals. Such voltage doubler circuits commonly employ an energy storing capacitor which becomes charged between pacer pulses such that at the time of occurrence of an output from the pulse generator, the voltage on the capacitor is added to the normal supply voltage to yield a resulting stimulating pulse which is approximately twice the battery potential.
Because in prior art arrangements the voltage doubling capacitor and the sensing amplifier are each coupled to the same electrode leads, difficulty has been encountered in ensuring that the energy storing capacitor is fully recharged prior to the expiration of the normal refractory period. This has been due to the fact that the charging circuit for the voltage doubling capacitor included the resistors defining the input impedance of the sense amplifier. For proper sensing without undue loading, it has been a requirement that the input impedance of the sensing amplifier be somewhere in excess of 20,000 ohms. With an effective impedance of this magnitude coupled in the charging circuit for the voltage doubler capacitor, it necessarily resulted in an inordinately large charge recovery time. Thus, the voltage difference appearing across the input of the sensing amplifier at the completion of the refractory period was sufficiently large to be interpreted by the system as a R-wave. Because of this, prior art pacer systems could have the pulse generator inhibited when, in fact, no real R-wave was being spontaneously produced by the heart muscle.
In accordance with the present invention, there is provided a unique switching mechanism which is coupled between the heart contacting electrode lead terminals, the input to the sensing amplifier and the output from the voltage doubler along with suitable control circuitry for allowing stimulation, selectively, at a first voltage or at approximately twice that voltage, while still isolating the sense amplifier from the pulse generating circuitry during a pacer pulse interval. Furthermore, the switching device of the present invention ensures that only a relatively low impedance will be connected in series between the voltage source and the voltage doubling capacitor for a predetermined period following the generation of a pacer pulse so that the capacitor becomes recharged well before the expiration of the refractory period. However, when the switching circuit is conditioned such that sensing of naturally occurring R-waves is to take place, the input impedance of the sensing amplifier is sufficiently high so as to not load down the signal source. Furthermore, the current which flows to recharge the voltage doubling capacitor following the generation of a pacer pulse is through the heart load and in a direction opposite to the flow during the period of the stimulating pulse. As such, the propensity toward iontrophoresis is reduced.